Here is my perfect world
Mac --> should be used by the regular home user. Macs are easy to use, have all the software a user will ever need incorportated in it. Although not cheap, Macs are built to last. The average user will never have a problem with a mac. The are also to be used by artists, photographers and video editors. Alos for school settings.
Linux --> To be used in workstations and servers. There are a lot less viruses for linux than windows. Linux provides stability and security that enterprises need. Programmers and software developers are to use Linux. Nerds should also use Linux.
Windows --> Windows has a lot of business features. it Should only be used for a business setting and/or school setting. Windows is a powerful operating system that provides many features businesses could use. There is also a plentiful number fo programs and software out there for windows. This is also a very tweakable system, so it meets the business' needs.
So there you have it, my perfect world.
The world we live in:
Mac --> Used by artists, photographers, video editors, and a handful of home users.
Linux --> It's starting to be used by companies, but not very much. Used mostly by nerds.
Windows --> Used by everyone. You to your school and what do they have? windows. You go to the public library, windows. You go to a restaurant, windows. It's taken over the world.